The sister of a woman who was brutally murdered at a farmhouse in Western Australia says her convicted killer should not have the right to apply for parole.

Key points:

William Mitchell killed Karen MacKenzie and her three children in 1993

After serving mandatory 20 years, can apply for release every three years

Karen's sister wants him to stay in jail, says having to fight his parole bid forces her to relive trauma

William Patrick Mitchell used an axe to kill Karen MacKenzie, 31, and her son and two daughters at their home in Greenough, 400 kilometres north of Perth, in 1993.

After serving his mandatory 20-year non-parole period, he is now allowed to apply for release every three years.

Mitchell is due to have his parole considered for the second time by the Prisoners Review Board in October.

Ms MacKenzie's sister Evalyn Clow said she is forced to relive the trauma of her sister's death each time the convicted killer is allowed to seek release.

"It just doesn't let me move on properly. I have to keep fighting every three years to keep him in jail," she said.

"It is very physically and mentally challenging to do this all the time, but it's something that has to be done."

Mitchell was originally sentenced to an indefinite term of imprisonment without parole, but that decision was overturned through an appeal.

Ms Clow said someone who had committed a horrific crime like Mitchell should not be given the option of freedom.

"He's not even 50. He's still young enough to go out and have a full life. He doesn't deserve that, he stole four lives," she said.

Push for parole laws to be changed

Ms Clow was diagnosed with Post Traumatic Stress Disorder after the loss of her sister, nephew and nieces, and has since seen physiatrists, psychologists and counsellors.

She said changes to the law were needed to keep criminals like Mitchell behind bars.

He deserves nothing because he took so much away from so many precious people.

Evalyn Clow

"I just hope one day in the future that families don't have to fight as hard as we do to keep any person in jail that does this sort of thing," Ms Clow said.

City of Greater Geraldton Mayor Shane Van Styn said the Greenough murders had left a deep scar in the community, and it was "outrageous" the convicted killer be allowed to apply for parole.

He called for State Parliament to be given the power to rule against Mitchell's right to apply for parole.

"I think there should be special dispensation created somewhere in the sentencing act that allows parliament to rule that a murderer of such a scale, or a person convicted of such a serious offence, is never released," he said.

"If the judiciary is unable to find it within themselves to ... execute such a sentence, then it certainly should be within the realm of parliament to be able to pass law that says the person should never be released.

Parole reviews necessary, Mischin says

Attorney-General Michael Mischin said there was no plan to make any changes to the current system for parole reviews.

In a statement, Mr Mischin said the periodic reviews were "considered necessary to ensure that no prisoner is 'lost' in the system".

But he said he would give due weight to observations made by Justice Owen who, in sentencing Mitchell, said "so far as I am concerned you will never be released ... if the [Parole] Board looks at it in the same way as I do it could be many, many years greater than 20 before you would merit consideration."